Acoustic sensor for pipeline deposition characterization and monitoring

ABSTRACT

A method and apparatus for analyzing a deposited layer on the inner surface of a fluid container wall having inner and outer surfaces are disclosed. One embodiment of the method comprises (a) transmitting an acoustic signal from a transmitter at a first distance from the outer surface of the wall; (b) receiving a first received signal A, comprising a reflection from the wall outer surface; (c) receiving a second received signal B, comprising a reflection from the wall inner surface; (d) receiving a third received signal C from the wall inner surface; (e) calculating a coefficient R wp  from A, B and C, and (f) calculating a coefficient R pd  from A, B and R wp  and calculating the acoustic impedance of the deposited layer Z d  from R wp , R pd , and Z w , where Z w  is the acoustic impedance of the material between the transmitter and the wall outer surface. A preferred embodiment of the apparatus comprises a piezoelectric or ferroelectric transducer having front and back faces; a backing member acoustically coupled to said transducer back face and impedance-matched to said transducer element, said backing member having proximal and remote faces; and a delay material disposed between said transducer front face and the wall outer surface.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] Not applicable.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

[0002] Not Applicable.

BACKGROUND OF THE INVENTION

[0003] As the current trend in offshore oil and gas production advancesinto deeper waters, it is becoming increasingly necessary for theindustry to develop cost effective solutions for developing fields indeep and/or remote waters.

[0004] A typical solution for such cases is to keep the productionfacilities on a “host platform” and connect the deep-water well(s) tothe platform with pipelines and risers. The supporting equipment for thesubsea tree control, such as hydraulic and electric power units,chemical injection pumps and tanks, and a control console, are alsohoused on the host platform. The subsea tree control is accomplished vialong umbilical(s) consisting of electric conductors, hydraulic lines andchemical injection lines laid alongside the pipeline. In addition, twoparallel pipelines are necessary to accomplish the roundtrip piggingoperations. The distance between the well and the host platform is knownas the tieback distance. The cost and technical challenges of this typeof conventional tieback system increase as the tieback distanceincreases, and to a lesser extent as the water depth increases. In mostcases, 20 miles represents the practical limit for the maximum tiebackdistance with the conventional tieback system.

[0005] One limit on the length of subsea tiebacks conveying crudepetroleum arises from flow assurance problems. Solids such as asphalteneand paraffin deposit on the inner walls of the tiebacks and partially,and in some cases completely, block the flow. The longer the tieback is,the greater the length of pipe that must be inspected and kept free ofdeposits.

[0006] At present, non-intrusive sensors that can adequately detect andcharacterize such deposits are not available. The present solutionsrequire use of very expensive alternative methods for flow assurance,including twin flowlines (for round-trip pigging), heat traced orinsulated tiebacks. These alternative methods operate by attempting toprevent the deposition of solids on the flowline wall, and do notprovide means for detecting the presence of solids in the event thatdeposits occur. The lack of continuous monitoring can result inundesirable shutdowns. For example, a flowline has been kept clear bypigging at a certain frequency, e.g. once per month, and the compositionof the fluid in the flowline changes so that deposits begin to form at agreater rate, the line will become clogged and possible shut downbecause the previously established pigging frequency is nowinsufficient.

[0007] Guided acoustic waves similar those described in U.S. Pat. No.5,892,162, have been used to detect corrosion in pipes based onreflections from corroded regions. Corrosion and scaling has also beendetected in insulated pipelines on surface using guided waves andliterature regarding this has been published from Imperial College,University of London.

[0008] Monitoring devices such as that described in U.S. Pat. No.4,490,679 identify paraffin by monitoring change in the resistance of anelectromagnetic coil. The monitoring device requires access to the fluidand is housed in a recess in the pipe. It is desired to providemonitoring without disrupting the flow of fluid through the line andwithout requiring direct contact with the fluid.

[0009] In U.S. Pat. No. 4,843,247, an optical asphaltene sensor isdescribed. This sensor determines the content of asphaltene in heavyoils, based on the absorption spectra of asphaltene. The invention usesvisible light in the region 500 nm to 1000 nm and thus requires at leastoptical access to the fluid. Furthermore, it does not distinguishbetween deposited and suspended asphaltene solids.

[0010] Similarly, ultrasonic longitudinal wave measurements have beenused to characterize fluids using reflectance methods, as in U.S. Pat.No. 4,571,693. Shear reflectance has been used in prior art to monitorcasting processes as in U.S. Pat. No. 5,951,163, detect viscosity as inU.S. Pat. No. 3,903,732, or density as in U.S. Pat. No. 5,886,250 and tomonitor the rheology of fluids.

[0011] Hence, it is desired to provide a system that can operate overgreater tieback distances without the cost and technical disadvantagesthat heretofore have prevented increasing the tieback distance. It isfurther desired to provide a method and apparatus for detecting andcharacterizing deposits of asphaltene, paraffin or hydrates on theinside wall of a pipeline. It is further desired to provide a systemthat can be installed on a conventional pipeline and does not impede theflow of fluid through the pipeline. The desired system should be able tocompensate for drift in the response of its components and should becapable of operating for a period of years without service orcalibration.

SUMMARY OF THE INVENTION

[0012] The present invention provides a method and apparatus that allowsnon-invasive monitoring of longer tieback distances without the cost andtechnical disadvantages associated with previous methods. The system ofthe present invention measures the acoustic properties of deposits onthe inner surface of the pipe wall. One object of the invention is todetect, characterize and determine the extent of deposition and thusenable remedial procedures.

[0013] The present system detects deposits or deposition of asphalteneand paraffin on the inside wall of a pipeline without impeding the flowof fluid through the pipeline. Furthermore, the present systemcompensates for drift in the response of its components and is thereforecapable of operating for a period of years without service orcalibration.

[0014] In particular, the present system includes an acoustic sensorthat is capable of detecting and characterizing deposits of paraffin,asphaltene or hydrates on the inner walls of pipes, thus enabling timelyintervention and flow assurance. In one embodiment, the sensor detectsand monitors deposition in a section of the pipe. In another embodiment,multiple installations of the system allow the location of depositionsto be determined with a desired degree of precision.

[0015] The present apparatus is capable of self-calibration and is notaffected by drifts in equipment response that may be caused byvariations in temperature or pressure or by the passage of time. Thepresent sensors distinguish between types of deposition material basedon the frequency and phase response.

[0016] In one embodiment, the present system is used to monitor andcharacterize the deposition and build-up of materials such as paraffin,asphaltene and hydrates in subsea tiebacks. Alternatively, the presentsystem can be permanently installed in a borehole to monitor depositiontherein. The present sensor can also be used on surface pipelines tomonitor deposition of solids in cases where solids deposition may occur,such as multiphase flow.

[0017] In a preferred embodiment, the sensor distinguishes the type ofdeposition material based on the compression and shear impedance as wellas signal arrival times.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] For a more detailed understanding of the invention, referencewill be made to the accompanying Figures, wherein:

[0019]FIG. 1 is a cross-sectional view of an apparatus according to afirst embodiment of the present invention mounted on a pipe;

[0020]FIG. 2 is a schematic diagram showing propagation and reflectionof an acoustic signal through the components of the apparatus of FIG. 1;

[0021]FIG. 3 is a representation of the signals received as a result ofthe reflections shown in FIG. 2; and

[0022]FIG. 4 is a cross-sectional view of an apparatus according to analternative embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0023] Two methods of measuring acoustic impedance of deposits based onacoustic reflectance (longitudinal or shear) are disclosed here. In thefirst method, deposition impedance is computed from the amplitude ofreflected acoustic signals arriving from a delay wedge/pipe wallinterface and the wall-deposition interface. The other methodadditionally measures acoustic reflection from the remote end of thetransducer backing and uses the reflected amplitude as a reference.

[0024] Transmitter-Receiver Arrangement

[0025] Referring initially to FIG. 1, a preferred embodiment of thepresent transducer system includes a piezoelectric or ferroelectrictransducer block 10, the wall of the pipe 30, and a layer of depositedsolid or semi-solid material 35, as shown in FIG. 1. Transducer block 10preferably comprises an impedance matched backing solid 12,piezoelectric or ferroelectric element (PZT) 14, and a delay wedge 16,which is preferably impedance matched with PZT 14. The outer end 13 ofbacking 12 is exposed to a fluid medium (such as air or water), whilethe inner end 15 is fixed to the outer face 17 of PZT 14. In onepreferred embodiment, backing solid 12 is designed so that the distancebetween ends 13, 15 is great enough to ensure that reflections fromouter end 13 will not overlap with the reflected signals from otherinterfaces, including the wedge 16/pipe wall 30 interface, and the pipewall 30/deposit 35 interface. The inner end 18 of PZT 14 is acousticallycoupled to delay wedge 16, which preferably has an acoustic impedanceclose to the acoustic impedance of PZT 14. The function of wedge 16 isto produce a reflection at the wedge-pipe wall interface, which helps incharacterizing the pipe wall, as discussed below. The delay wedge 16preferably comprises of titanium or alloys of titanium with acousticimpedance close to the acoustic impedance of PZT 14

[0026] It is necessary to calibrate reflection coefficients in order toaccurately measure the impedance of the deposit 35 (longitudinal and/orshear). In particular, temperature and material property variations thatcause pipe wall impedance variations must be compensated for. Animplicit compensation method is discussed in the following paragraphs.

[0027] Delay wedge 16 is preferably constructed of an elastic material.Its temperature-dependent longitudinal and shear impedance are known,e.g. from lab measurements. The deposited materials 35 are typicallyvisco-elastic in nature.

[0028] A clamp or retaining device 50 is used to maintain good acousticcoupling between transducer block 10 and the pipe wall 30. One or moreBelleville springs 52 or other biasing means may be positioned betweenclamp 50 and transducer block 10 to urge block 10 toward pipe 30. Clamp50 may be held together with the pipe by a threaded sleeve 54, whereinthe end of the clamp 50 mates with the rising section of sleeve 54.Sleeve 54 clamps circumferentially on the outside of the pipe. Clamp 50could be designed to allow pressure balancing.

[0029] Referring now to FIGS. 2 and 3, PZT 14 emits an incident acousticsignal P₀. A portion of the signal P₀ is reflected back from each of theinterfaces across which it travels. Thus, PZT 14 measures severalreflections from various interfaces. These signals are labeled in FIGS.2 and 3 as A—reflection from the delay wedge 16/wall 30 interface,B—first reflection echo from the wall 30/deposit 35 interface, and Csecond reflection from the wall 30/deposit 35 interface. If Z_(w) andZ_(p) are the (either longitudinal or shear) impedances of the wedge andwall, respectively, the reflection coefficient of the wedge/wallinterface R_(wp) can be written as$R_{w\quad p} = \frac{Z_{p} - Z_{w}}{Z_{p} + Z_{w}}$

[0030] R_(wp) can also be written in terms of the three reflectedsignals A, B and C, as$R_{w\quad p} = \sqrt{\frac{A \cdot C}{{A \cdot C} - B^{2}}}$

[0031] The impedance Z_(w) of delay wedge 16 is known and depends on theselection of the wedge material. Hence, the coefficient R_(wp) can bedetermined from A, B and C and used to compute Z_(p).

[0032] The reflection coefficient of the wall 30/deposit 35 interfaceR_(pd), can be given by$R_{p\quad d} = {\frac{B}{A}\frac{R_{w\quad p}}{\left( {1 - R_{wp}^{2}} \right)}^{2L_{p}\alpha_{p}}}$

[0033] where, L_(p) and α_(p) are the thickness and attenuation of thepipe wall, respectively. Once R_(wp) and R_(pd) are determined, theacoustic impedance of the deposited layer 35, Z_(d) is preferablydetermined by$Z_{d} = {Z_{w}{\frac{1 + R_{wp}}{1 - R_{wp}} \cdot \frac{1 + R_{pd}}{1 - R_{pd}}}}$

[0034] where, Z_(w) is the impedance (longitudinal or shear) of delaywedge 16.

[0035] In this manner, the longitudinal and/or shear impedance of thedeposited material (e.g. wax) can be determined from the measurableamplitude of the reflected delay wedge 16/wall 30 echo A and theamplitudes of the first and second reflected echoes B and C.

[0036] One advantage provided by the present invention is that, indetermining the acoustic impedance of deposition, it avoids the need touse reference signals that may be generated by acoustic delay lines withnotches, slots and holes as reflectors such as are commonly used in theprior art, such as U.S. Pat. No. 4,571,693. Notches and slots introduceundesirable non-uniform scattering of the acoustic waves. In addition,the notched or slotted delay lines used in the art require carefulhandling during construction because of their reduced strength.

[0037] In an alternative approach, reflected acoustic waves from theouter end 13 of backing 12 are used as a reference signal A_(ref) (FIG.2). This method is useful in the circumstances where the secondwall/deposit reverberation signals are either weak or overlap with thefirst wall/deposit echo. In this approach, backing solid 12 ispreferably relatively long. For instance, the length of backing solid12, L_(b), preferably equals at least six times the total length ofdelay wedge 16 L_(w) plus the wall thickness L_(p), i.e.L_(b)≧6(L_(w)+L_(p)). This enables the reference signal to arrive latercompared to reflections from other interfaces. The far side of thebacking may be exposed to an unchanging media, preferably air, in orderto maintain a constant reflection coefficient. The pipe wall impedanceZ_(p) is given as$Z_{p} = {\frac{\left. {1 -} \middle| \frac{A}{A_{ref}} \right|}{\left. {1 +} \middle| \frac{A}{A_{ref}} \right|}Z_{w}}$

[0038] where A is the reflection from the delay wedge/wall interface,and A_(ref) is the reference signal from the outer face of the backingsolid. Then, deposition impedance can be calculated from the absolutevalues of B and A_(ref), for small attenuation of pipe wall material, as$Z_{d} = {\frac{\left. {1 - \frac{\left( {Z_{w} + Z_{p}} \right)^{2}}{4Z_{w}Z_{p}}} \middle| \frac{B}{A_{ref}} \right|}{\left. {1 + \frac{\left( {Z_{w} + Z_{p}} \right)^{2}}{4Z_{w}Z_{p}}} \middle| \frac{B}{A_{ref}} \right|}Z_{p}}$

[0039] where B is the first reflection echo from the wall 30/deposit 35interface,

[0040] ). Therefore, from the above two approaches, when alongitudinal-wave or a shear-wave transducer is used, longitudinal orshear impedance of the deposition, i.e., density×longitudinal (or shear)speed of sound, can be measured. Deposition material (as paraffin,asphaltene, hydrates) are usually regarded as visco-elastic materialwith both bulk module and shear module. The shear impedance consists ofa real part and an imaginary part. The real part (density×shear speed ofsound) can be determined from the above approaches. The imaginary partof the shear impedance, which is a product of the viscosity and densityof the deposited material and the wave frequency, can be determinedseparately from measurement of the phase shift of the reflectioncoefficient due${R \cdot ^{i\quad \varphi}} = \frac{Z_{p\quad s} - \sqrt{\omega\rho\eta}}{Z_{p\quad s} + \sqrt{\omega\rho\eta}}$

[0041] where,

[0042] to the deposit as follows; φ is the phase difference between thethe incident and reflected signals,

[0043] R is the absolute reflection coefficient,

[0044] Z_(ps) is the shear impedance of the pipe wall

[0045] These measurements provide the acoustic longitudinal and shearimpedance, and phase shift of the acoustic waves due to visco-elasticityof the deposition. This information is combined to characterize the typeof deposit based on the measured longitudinal (and shear) impedance ofthe deposit.

[0046] The present invention provides several advantages over prior artsystems. These include but are not limited to:

[0047] non-invasive and non-intrusive detection, identification,characterization and monitoring of deposits in real-time.

[0048] low assurance monitoring and assessment of intervention based ondeposit characteristics.

[0049] quantitative monitoring of deposits in critical areas.

[0050] compensation for variation in signal amplitude over a prolongedperiod of time (signal drift).

[0051] potential for use with existing tubing (retrofitting).

[0052] In a preferred embodiment, a primary application of the presentsystem is to monitor and characterize deposition and build-up ofmaterials such as paraffin, asphaltene, hydrates and infiltrated sand insubsea tiebacks. The present system can also be used to advantage insmart wells, where it is permanently installed in a borehole andinterfaced with a microprocessor to monitor deposition. This sensor canalso be used on surface pipelines to monitor deposition of solids inmultiphase flow.

[0053] Compression acoustic impedance is a function of the layer densityand speed of sound. Shear acoustic impedance is a function layer densityand viscosity. The phase of the acoustic reflections depends on thedamping properties (viscosity in oil and visco-elasticity in asphaltene,paraffin, etc.) of the deposit. Thus, based on the effect of thedeposition layer on the compression and shear wave reflectance, aninverse solution calculates the deposition layer properties andidentifies composition.

[0054] In an alternative embodiment shown in FIG. 4, the acoustic sensorcomprises both a compression-piezoelectric element 24 and ashear-piezoelectric element 34. A pressure clamp like clamp 50 of FIG. 1is preferably used, but is not shown in FIG. 4. Piezoelectric elements24, 34 are preferably bonded together and have a common-groundelectrode. The relative positions of the shear and compression elementscan be reversed from those shown in the Figure. The back of the upperelement is preferably coupled to impedance-matched backing 12. Anelastomer 26 preferably acoustically couples the element to the pipewall. This construction is advantageous because the probe interrogatesthe same layer of deposition with both compression and shear waves.

[0055] The following are some preferred embodiments of the invention:

[0056] an acoustic transducer that can be clamped on the exterior ofexisting pipe, consisting of impedance-matched backing solid, apiezoelectric or ferroelectric transducer element, and a delay wedge.The far end of the backing is exposed to a fluid medium, while the nearend is fixed to the frontal face of the piezoelectric element. Thetransducer system is capable of compensating for the pipe wall materialproperty variations by measuring multiple reflections from the pipe walland far end of the backing solid.

[0057] an acoustic device capable of generating and detectingcompression and/or shear acoustic waves, which reflect from severalreflecting interfaces including the interface between the pipe wall anddeposits on the inner walls of pipes that are transporting crudepetroleum.

[0058] an active acoustic sensor capable of characterizing the type ofdeposition on the inner wall of pipes, based on frequency-dependentphase and amplitude information in the reflected acoustic waves.

[0059] an active acoustic sensor capable of estimating thickness of thedeposition and thus monitoring the layer buildup, based on the arrivaltime of the reflected wave from the deposit/fluid interface.

[0060] an acoustic sensor capable of monitoring deposition layer buildupand triggering alarms for remedial action in case the depositionthickness exceeds a predetermined thickness.

[0061] an acoustic wave sensor that is capable of compensating forvariation in signals over a period of time by using reflections from thefar end of the backing material as a reference.

[0062] While preferred embodiments of the present invention have beendisclosed and discussed herein, it will be understood that variousmodifications can be made to these embodiments without departing fromthe scope of the invention. For example, the principles described hereincan be used to determine the presence and nature of buildup or depositson walls other than pipeline walls, including but not limited tocontainer walls. The present apparatus can be used to detect buildup ordeposits on inner or outer walls, depending on how the apparatus isused. The dimensions and/or relative proportions of the components ofthe apparatus can be modified, as can the number and frequency ofsignals that are emitted, detected and/or analyzed by the apparatus. Inthe claims that follow, any recitation of steps is not intended as arequirement that the steps be performed sequentially, or that one stepbe completed before another step is begun, unless explicitly so stated.

What is claimed is:
 1. A method for analyzing a deposited layer on theinner surface of a fluid container wall having inner and outer surfaces,comprising: (a) transmitting an acoustic signal from a transmitter at afirst distance from the outer surface of the wall; (b) receiving a firstreceived signal A, comprising a reflection from the wall outer surface;(c) receiving a second received signal B, comprising a reflection fromthe wall inner surface; (d) receiving a third received signal C from thewall inner surface; (e) calculating a coefficient R_(wp) from A, B and Cusing the equation$R_{w\quad p} = \sqrt{\frac{A \cdot C}{{A \cdot C} - B^{2}}}$

(f) calculating a coefficient R_(pd) from A, B and R_(wp) using theequation$R_{p\quad d} = {\frac{B}{A}\frac{R_{w\quad p}}{\left( {1 - R_{wp}^{2}} \right)}^{2L_{p}\alpha_{p}}}$

where L_(p) is the thickness of the wall and α_(p) the attenuation ofthe wall; and (f) calculating the acoustic impedance of the depositedlayer Z_(d) using the equation${Z_{d} = {Z_{w}{\frac{1 + R_{wp}}{1 - R_{wp}} \cdot \frac{1 + R_{pd}}{1 - R_{pd}}}}},$

where Z_(w) is the acoustic impedance of the material between thetransmitter and the wall outer surface.
 2. The method according to claim1, further including providing an acoustic delay material between thetransmitter and the wall outer surface.
 3. The method according to claim1 wherein the acoustic impedance of the deposited layer Z_(d) calculatedin step (f) is a longitudinal impedance.
 4. The method according toclaim 1 wherein the acoustic impedance of the deposited layer Z_(d)calculated in step (f) is a shear impedance.
 5. The method according toclaim 1, further including receiving a fourth received signal from thedeposit/fluid interface and using said received signals to estimate thethickness of the deposited layer.
 6. The method according to claim 1wherein the signal transmitted in step (a) comprises a shear wave,further comprising transmitting a second signal that comprises acompression wave.
 7. A method for analyzing a deposited layer on theinner surface of a fluid container wall having inner and outer surfaces,comprising: (a) transmitting an acoustic signal from a transmitter at afirst distance from the outer surface of the wall using a transmitteracoustically coupled to a backing, said backing including a first endproximal to the transmitter and a second end remote from thetransmitter; (b) receiving a first received signal A, comprising areflection from the wall outer surface; (c) receiving a reference signalA_(ref), comprising a reflection from the backing second end; (d)receiving a second received signal B, comprising a reflection from thewall inner surface; (e) calculating an impedance for the wall materialusing the equation${Z_{p} = {\frac{\left. {1 -} \middle| \frac{A}{A_{ref}} \right|}{\left. {1 +} \middle| \frac{A}{A_{ref}} \right|}Z_{w}}};{a\quad n\quad d}$

(f) calculating the acoustic impedance of the deposited layer Z_(d)using the equation$Z_{d} = {\frac{\left. {1 - \frac{\left( {Z_{w} + Z_{p}} \right)^{2}}{4Z_{w}Z_{p}}} \middle| \frac{B}{A_{ref}} \right|}{\left. {1 + \frac{\left( {Z_{w} + Z_{p}} \right)^{2}}{4Z_{w}Z_{p}}} \middle| \frac{B}{A_{ref}} \right|}{Z_{p}.}}$


8. The method according to claim 7 wherein the length of the backingbetween the backing first end and the backing second end is at least sixtimes the total of the first distance from the outer surface of the wallplus the wall thickness.
 9. The method according to claim 7, furtherincluding providing an acoustic delay material between the transmitterand the wall outer surface.
 10. The method according to claim 7, furtherincluding receiving a third received signal from the deposit/fluidinterface and using said received signals to estimate the thickness ofthe deposited layer.
 11. A method for analyzing a deposited layer on theinner surface of a fluid container wall having inner and outer surfaces,comprising: (a) providing a first piezoelectric or ferroelectrictransducer having front and back faces and transmitting a shear waveinto said container wall from said first transducer; (b) providing asecond piezoelectric or ferroelectric transducer having front and backfaces and transmitting a compression wave into said container wall fromsaid second transducer;
 12. The method according to claim 11, furtherproviding a backing member acoustically coupled to the back face of atleast one of said transducers, said backing member having proximal andremote faces, said backing member having proximal and remote faces. 13.The method according to claim 11, further including providing a delaymaterial disposed between the front face of at least one of saidtransducers and the wall outer surface.
 14. The method according toclaim 11, further including, for at least one of said transmittedsignals, the steps of (c) receiving a first received signal A,comprising a reflection from the wall outer surface; (d) receiving areference signal A_(ref), comprising a reflection having a known delayperiod; and (e) receiving a second received signal B, comprising areflection from the wall inner surface.
 15. The method according toclaim 14, further including the steps of: (f) calculating an impedancefor the wall material using the equation${Z_{p} = {\frac{\left. {1 -} \middle| \frac{A}{A_{ref}} \right|}{\left. {1 +} \middle| \frac{A}{A_{ref}} \right|}Z_{w}}};{a\quad n\quad d}$

(g) calculating the acoustic impedance of the deposited layer Z_(d)using the equation$Z_{d} = {\frac{\left. {1 - \frac{\left( {Z_{w} + Z_{p}} \right)^{2}}{4Z_{w}Z_{p}}} \middle| \frac{B}{A_{ref}} \right|}{\left. {1 + \frac{\left( {Z_{w} + Z_{p}} \right)^{2}}{4Z_{w}Z_{p}}} \middle| \frac{B}{A_{ref}} \right|}{Z_{p}.}}$


16. An acoustic device for measuring buildup on a container wall havinginner and outer surfaces, comprising: a piezoelectric or ferroelectrictransducer having front and back faces; a backing member acousticallycoupled to said transducer back face and impedance-matched to saidtransducer element, said backing member having proximal and remotefaces; and a delay material disposed between said transducer front faceand the wall outer surface.
 17. The device according to claim 16 whereinthe device characterizes the buildup based on frequency-dependent phaseand amplitude information in the reflected acoustic waves.
 18. Thedevice according to claim 16 wherein the distance between the proximaland remote backing faces is at least six times the distance between thetransducer front face and the wall inner surface.
 19. An acoustic devicefor measuring buildup on a container wall having inner and outersurfaces, comprising: a first piezoelectric or ferroelectric transducerhaving front and back faces; and a second piezoelectric or ferroelectrictransducer having front and back faces; one of said first and secondtransducers being capable of generating shear waves in the containerwall and the other of said first and second transducers being capable ofgenerating compression waves in the container wall
 20. The deviceaccording to claim 19, further including a backing member acousticallycoupled to the back face of one of said transducers andimpedance-matched to said transducer.
 21. The device according to claim19, further including a delay material disposed between the front faceof one of said transducers and the wall outer surface.
 22. The deviceaccording to claim 19 wherein said first transducer is disposed betweensaid second transducer and said container wall.
 23. The device accordingto claim 22, further including a backing member acoustically coupled tothe back face of said second transducer and impedance-matched to saidsecond transducer.
 24. The device according to claim 22, furtherincluding a delay material disposed between the front face of said firsttransducer and the container wall.
 25. The device according to claim 22,further including an elastomeric material disposed between the frontface of said first transducer and the container wall.
 26. The deviceaccording to claim 19 wherein the device characterizes the buildup basedon frequency-dependent phase and amplitude information in the reflectedacoustic waves.